1. Atmosphere-Ocean Interaction

2. Upcoming Schedule Monday, March 17: finish Chapter 10 + office hours
Wednesday, March 19: Second midterm exam. The exam will cover Chapters 6, 7, 8, 9 and 10.
Closed book
No textbooks, calculators or cheatsheets.
Alternate (anti-social) seating
Bring a picture ID to the exam.
Remember to:
Write the 5-digit test code in lines 76-80
Write your name on the exam sheet and sign it
Turn in both the exam sheet and the answer sheet.
Friday, March 21: begin Chapter 11.

3. The Single Cell Model
This is a very simplified model based on the following three assumptions:
The Earth’s surface is uniformly covered with water (no differential heating of the land and the oceans)
The sun is always directly over the equator (no seasonal variations of the winds).
The Earth does not rotate.
No Coriolis effect.
The only active force is the pressure gradient force.

4. The Three Cell Model Keep two of the assumptions, relax the third:
The Earth is covered with a continuous ocean
The sun is always directly over the equator
The Earth rotates -> Coriolis force!

5. Observing global winds
Intertropical convergence zone
Observing global winds
from space

6. Winds and pressure in the real world
Semi-permanent highs and lows: persist throughout the year, correspond to converging/diverging upper air masses.
Bermuda, Pacific highs; Icelandic, Aleutian lows
Seasonal highs and lows (continents heat/cool faster)
Winter: Siberian high, Canadian high
Summer (thermal lows): Southwest US, Iran
January
July
Subtropical highs
Subtropical highs

7. Does it work?
Yes: semi-permanent highs and lows: persist throughout the year
No: seasonal highs and lows
(continents heat/cool faster)
No: seasonal shifts N-S (today)
No: winds aloft in mid latitudes
January
July
Subtropical highs
Subtropical highs

8. Effect on the local climate
During summer, the Pacific high moves north near the CA coast, and the Bermuda high moves closer to the SE coast.
CA: dry summers
GA: wet summers

9. Seasonal Effects on the Global Circulation.
January
The three cell model neglects the Earth’s tilt
The tilt would cause seasonal N-S shifts
Bright white colors correspond to stronger winds.
The winds are stronger in the winter.
The wind system shifts slightly north-south during the year (think of bird migration)
July

10. The General Circulation and Precipitation Paterns
Converging surface flows:
Low surface pressure
Uprising air
Heavy precipitation
Diverging surface flows:
High surface pressure
Sinking air
Dry climate
Seasonal variations of the climate. Example: Timbo, Guinea (Chapter 17).
Timbo

11. Example: Tropical wet-and-dry climate (Aw)
Timbo (Guinea)
Wet summer season
Dry season (Dec-Feb): hot, desert-like conditions
Grassland savanna

12. Winds and Pressure Systems Aloft
The wind system aloft differs from the surface wind system. It is close to a geostrophic flow.
There is no significant friction with the ground.
The three cell model does not work that well in the middle latitudes.
The winds aloft are stronger than on the ground.
In the winter the gradients are bigger -> the winds are stronger.
July
January

13. Winds Aloft
Warm air above the equator and cold air above the polar regions
Higher pressure at the equator, lower pressure both to the north and to the south of the equator
The pressure gradient force is towards the poles, sets the air
in motion
The Coriolis force
NH: to the right
SH: to the left
The wind turns right in the NH and left in the SH, becomes parallel to the isobars
Westerly winds aloft in both the NH and SH.
Easterly winds at the surface in both the NH and SH.

14. Jet Streams Swiftly flowing air currents aloft Tropopause jets
Subtropical: 13km high, 30N
Polar: 10km high, 60N
Strong westward winds
Meandering pattern
They result from strong temperature
gradients at
those latitudes
(hence strong
PG forces)

15. Tropopause jet streams
Discontinuous. Position varies daily.
May merge or split (northern and southern branches)
Meandering pattern: may flow to the north or south
Important for the global heat redistribution
Affect local weather conditions

16. Atmosphere-Ocean Interaction
The surface of the Earth impacts the air motion through the friction force.
Newton’s III law: for every action there is a reaction. The atmosphere exerts a friction (drag) force on the surface of the Earth.
The prevailing surface
winds drive the surface
currents in the oceans.
Variations in the surface
temperature of the ocean
impacts the atmospheric T
as well as amount of the
water vapor in the
atmosphere.

17. Major ocean currents Gyres: circular whirls of ocean currents
25-40 deg angle b/n wind and current
Clockwise in the North Pacific and Atlantic
Counterclockwise in the South Pacific and Atlantic

18. Ekman Spiral The surface winds drive the surface currents.
The friction force acting on the ocean surface
is in the direction of the wind.
The Coriolis force deflects the current to the right (NH).
Each layer of water drags
the water layer below, and
the Coriolis force pushes
the moving water below
to the right.
The direction of the ocean
current changes with depth.
The net mass transport within the
first 100 m is perpendicular
to the wind direction.

19. Upwelling of the Ocean Water
Winds plowing along the coastal line result in a net surface current perpendicular to the wind and away from the coast.
The warm surface water is
replaced by an upwelling
deep cold water.
Water upwelling explains
the low water temperatures
along the California coast.

20. Surface Water Temperature Along California’s Coast
The low water temperatures result in low saturation vapor pressure over the ocean.
Recall the advection fog example from Chapter 5
As the air moves over
the land the air warms
up and results in low
relative humidity.
The summer is very dry.